Cytochrome b5, an intrinsic membrane protein of the endoplasmic reticulum, can be isolated from rabbit liver by detergent solubilization. The protein is water soluble and exists in aqueous solution as a mixture of monomers and octomers. We have previously shown that cytochrome b5 binds rapidly to lipid vesicles but will also exchange between vesicles. This exchange process may be relevant to the role of cytochrome b5 in lipid desaturation processes and also to the intramembrane distribution of proteins in the cell. The mechanism of cytochrome b5 exchange between lipid vesicles and between natural membranes will be examined. Exchange from one vesicle to another will be monitored by incorporation of a fluorescent quenching agent in one vesicle population. As the protein enters this vesicle, tryptophan fluorescence in the hydrophobic tail will be quenched. With this assay the effect of lipid composition, temperature and salt concentration on the exchange process will be monitored. The rates of exchange will be correlated with rates of cytochrome b5 binding to vesicles in order to elucidate the mechanism of the exchange process.